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Description of Habitat: The Yampa River is the largest remaining free-flowing river in the Colorado River basin (Carlson and Muth 1989). The Yampa River flows 198 miles and drops about 7,445 feet in elevation (Tyus and Karp 1989). Only minimal physical and hydrologic changes have occurred in the mainstream Yampa River. The most significant habitat alteration may have been the introduction of nonnative fishes into the Yampa River; nonnative fishes represent 15 of 24 species reported by Tyus et al. (1982a). From Cross Mountain Canyon to the Little Snake River, the Yampa River has boulder/cobble/gravel substrates. The river is fairly wide here, with riffles, eddies, and side channels common. Though there is at least one deep pool, the river is generally very shallow. The Yampa River receives waters from the Little Snake River near Deerlodge Park. Here, the substrate changes to sand and silt, mostly from the Little Snake River. The habitat is not varied, with few riffle, eddies, or backwaters. At RM 45, the Yampa abruptly enters Yampa Canyon. Here, steep canyon walls house the slow-moving river. Occasional boulder fields create rapids, but the predominant substrate is gravel/cobble with patches of sand. The river gradient increases in the lower portion of the canyon and the river meanders through soft sandstone cliffs. The Yampa River comes out of Yampa Canyon at Echo Park where it joins the Green River (Miller et al. 1982b). Relationship to Constituent Elements: Water The Yampa River has minimal water development, thus, when it is compared to other rivers in the basin, the current hydrograph reflects flows which are representative of historical volume and timing. The Service has identified existing Yampa River flows as adequate for the needs of native fishes (Tyus and Karp 1989). Yampa River flows have also been identified as critical for maintaining native fish habitat in the Green River below their confluence (McAda and Wydoski 1980; Tyus and Karp 1989). Water quality has not been identified as a chronic problem. However, an oil spill in June of 1989 caused by a ruptured pipeline discharged approximately 10,000 gallons into the Yampa River. This spill has been implicated in the decline of Colorado squawfish larval and young-of-year abundance in the upper Green River that year (Obmasick 1989; Carlson 1991), impacts to other species may have also occurred. Physical Habitat Data collected in recent years indicate that this reach of the Yampa provides spawning and adult habitat for the razorback sucker (Tyus and Karp 1989, 1990). Analysis of this reach by the Service (Miller et al. 1982b) shows principal habitat types to be pool, riffle, eddy, and side channel. Backwaters are an important habitat component in the lower portion of this reach, and rapids occur in Dinosaur National Monument. Backwaters also are a 46